The humoral immunity of patients with malignant tumors of the central nervous system is being studied using an established in vitro microtoxicity assay. A longitudinal study using both autologous and allogeneic tumor cells is under way to determine the frequency of complement dependent cytotoxic antibodies in patients with varying tumor loads. Sera from these patients will be fractionated into IgG, IgM and IgA classes to determine the immunoglobulin class of the cytotoxic antibodies and how the class might alter with changes in the biological behavior of the tumor. Patients with other tumors and non-neoplastic central nervous system disease as well as normal persons will be tested for the presence of these antibodies. Serial absorptions were carried out to determine the specificities of the cytotoxic response. The methodical examination of the humoral response of patients with central nervous system tumors will lead to refined techniques for the immunodiagnosis and serological monitoring of these patients.